Ultraviolet radiation-curable silicone release compositions

ABSTRACT

Novel UV-curable silicone release coating compositions and photocatalysts therefor are provided which include hydrogen-functional and vinyl-functional photoreactive terpolymers, which are especially compatable with certain perbenzoate ester photocatalysts. The addition of certain polyaromatic photosensitizers to enhance cure is also disclosed.

This application is a division of application Ser. No. 757,105, filed07/19/85 now U.S. Pat. No. 4,608,312, which is a division of Ser. No.527,299 filed 08/26/83, now U.S. Pat. No. 4,558,147.

FIELD OF THE INVENTION

This invention relates to ultraviolet radiation-curable silicone releasecompositions. More particularly, it relates to new photocurable siloxanepolymers and new photocatalyst-siloxane polymer combinations.Mercaptoalkoxyalkyl-functional polysiloxanes and vinyl-functionalsiloxane terpolymers have been discovered which are curable on exposureto ultraviolet radiation in the presence of a photoinitiator.

BACKGROUND OF THE INVENTION

Silicone compositions have become widely accepted as release coatings,which are useful to provide a surface or material which is relativelynonadherent to other materials which would normally adhere closelythereto. Silicone release compositions may be used as coatings whichrelease pressure sensitive adhesives for labels, decorative laminates,transfer tapes, etc. Silicone release coatings on paper, polyethylene,Mylar®, and other such substrates are also useful to provide non-sticksurfaces for food handling and industrial packaging.

Previously developed silicone release products have been heat-curable,however, the high energy costs and safety and environmentalconsiderations associated with high temperature oven curing has providedthe incentive for developing alternative technologies, such asultraviolet radiation-curable silicones.

Ultraviolet (UV) radiation is one of the most widely used types ofradiation because of its low cost, ease of maintenance, and lowpotential hazard to industrial users. Typical curing times are muchshorter, and heat-sensitive materials can be safely coated and curedunder UV radiation where thermal energy might damage the substrate.

Three basic UV-curable silicone systems have been developed:Epoxysilicone systems, acrylic-functional silicone systems, andmercaptofunctional silicone systems.

Epoxysilicone systems, such as those described in U.S. Pat. No.4,279,717 (Eckberg et al.) and commonly assigned copending U.S.application Ser. No. 375,676, filed May 6, 1982, now U.S. Pat. No.4,576,999, feature epoxy-functional diorgano siloxane diorganosiloxanebase polymers catalyzed by onium salt photoinitiators. The compositionsexhibit extremely high cure rates but depend on expensive materialswhich are not widely available. Acrylic-functional silicone systems suchas described in commonly assigned copending U.S. Applications Ser. Nos.239,297, filed Mar. 2, 1981, now U.S. Pat. No. 4,348,454, and 375,676,filed May 6, 1982, and also modified systems described in U.S. Pat. No.4,048,036 (Prucnal) and U.S. Pat. No. 4,017,652 (Gruber), provideserviceable coatings that are UV-curable in the presence of freeradical-type photoinitiators but require complex, multistep preparation.

Many mercapto-functional systems are known: For example, U.S. Pat. No.4,064,027 (Gant), U.S. Pat. No.4,107,390 (Gordon et al.), U.S. Pat. No.4,197,173 (Curry et al.), Japan Kokai Tokkyo Koho 79 48,854 (toTakamizawa; Chem. Abstracts 91: 58888r, 1979), U.S. Pat. No. 3,661,744(Kehr et al.), U.S. Pat. No. 4,070,526 (Colquhoun et al.), U.S. Pat. No.4,052,529 (Bokerman et al.), U.S. Pat. No. 3,873,499 (Michael et al.),and U.S. Pat. No. 3,816,282 (Viventi) disclose mercapto-functionalpolysiloxane or polythiol compositions which are UV-curable whencombined with an ethylenically unsaturated organic compound, whichcurable compositions also contain, variously, mercaptoalkyl polysiloxanecure accelerators, acetophenone-type photosensitizers, silacyclopentenylcuring agents, cure rate accelerators and gellation inhibitors. Theseprovide a wide range of serviceable release coating compositions,however, the acceptance of this technology has been hindered by severalpersistent disadvantages including dependence on scarce or expensivestarting materials, unserviceably slow curing rates, complex processing,and offensive odors (associated with the mercaptan group) which persistin the cured products. Some of these specific disadvantages have beenaddressed, for example, in the Gant patent, photosensitizers such asacetophenone are added to assist radiation curing, in U.S. 4,171,252(Fantazier), photopolymerization of unsaturated compounds is catalyzedby peroxy-napthalenic compounds, and in the aforementioned Kehr et al.and Colquhoun et al. patents aromatic ketones are employed to acceleratethe cure; however, there is still a need for improvement and reductionof costs in mercaptofunctional compositions and related UV-curetechnologies.

New mercaptoalkoxyalkyl-functional silicones have now been discoveredwhich can be synthesized in a two-step, one-vessel process from readilyavailable, inexpensive materials and which do not emit disagreeableodors. In addition, it has been discovered that certain perbenzoateesters are suitable photoinititators for polymerization reactionsbetween mercaptofunctional compounds and vinyl-functional compounds; andthe cure characteristics of the novel reactivepolysiloxane/photoinitiator blends of the present invention may beenhanced or modified by certain aromatic photosensitizers or byjudicious section of the molar ratios of reactive incredients in suchblends.

All of the patents and patent applications mentioned above are herebyincorporated by reference.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide newmercaptofunctional silicone release coating compositions.

It is a further object of the present invention to provide solventlesssilicone release coating compositions which are curable on briefexposure to ultraviolet radiation.

It is a further object of the present invention to provide improvedreactive polysiloxane/photoinitiator blends which are simply andinexpensively produced.

These and other objects are accomplished herein by amercaptoalkoxyalkyl-functional polysiloxane capable of curing (i.e.,polymerizing or crosslinking) to an abhesive polymeric composition onbrief exposure to ultraviolet radiation when combined with an organiccompound containing ethylenic unsaturation and a catalytic amount of aphotoinitiator comprising: A diorganopolysiloxane comprising units ofthe formula RR'SiO, where R is hydrogen or C.sub.(1-8) alkyl and R' ishydrogen, C.sub.(1-8) alkyl or a monovalentmercaptoalkoxyalkyl-functional organic radical of from 2 to 20 carbonatoms, said diorganopolysiloxane having up to about 50%mercaptoalkoxyalkyl-functional groups and a viscosity of from about 50to 10,000 centipoise at 25° C.

Also contemplated are mercaptoalkoxyalkyl-functionaldiorganopolysiloxanes prepared by (1) reacting adialkylhydrogen-chainstopped polydialkyl-alkylhydrogensiloxane copolymerwith an ω-halo-alkene in the presence of a precious metal hydrosilationcatalyst, and (2) reacting the product of (1) with a hydroxy-functionalthiol of the formula HO--R"--SH, where R" is a divalent alkylene oralkyl ketone radical of from 2 to 20 carbon atoms in the presence of anamine.

Another feature of the present invention is a UV-curable siliconerelease composition comprising:

(A) A diorganopolysiloxane comprising units of the formula RR'SiO, whereR is hydrogen or C.sub.(1-8) alkyl and R' is hydrogen, C.sub.(1-8) alkylor a monovalent mercaptoalkoxyalkyl-functional organic radical of from 2to 20 carbon atoms, said diorganopolysiloxane having up to about 50% byweight mercaptoalkoxyalkyl groups and a viscosity of from about 50 to10,000 centipoise at 25° C.;

(B) A polysiloxane consisting of from about 0.5 to 100 mole percent ofvinyl-functional siloxane units of the formula (CH₂ ═CH)R_(n)SiO_(3-n/2), where R is hydrogen or C.sub.(1-8) alkyl and n has a valueof 0 to 2, inclusive, any non-vinyl-containing siloxane units having theformula R³ _(m) SiO_(4-m/2), where R³ is hydrogen or C.sub.(1-8) alkyland m has a value of from 0 to 3, inclusive; and

(C) A catalytic amount of a photoinitiator.

Especially contemplated are UV-curable compositions wherein thephotoinitiator component is a perbenzoate ester; and further featuresinclude the use of certain aromatic ketones as photosensitizers toassist curing, and the discovery of a vinyl-functional polysiloxaneterpolymer which is useful in forming UV-curable release compositions.

Further embodiments of the present invention will become apparent tothose skilled in the art upon consideration of the followingdescription, examples, and claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides mercaptoalkoxyalkyl-functional siliconecompositions and silicone release coatings made from such compositions,as well as photoinitiators for such compositions and processes forproviding the compositions and coatings.

The mercaptofunctional polymers of the present invention arediorganopolysiloxanes comprised of siloxane units having substituentgroups including hydrogen atoms, lower alkyl radicals having up to aboutcarbon atoms such as methyl, ethyl, propyl, isopropyl, etc., andmonovalent mercaptoalkoxyalkyl radicals of from about 2 to 20 carbonatoms. These polymers may be advantageously synthesized from a number ofconstituent ingredients. The relative proportions of these constituentsare not critical and may be varied over a wide range to providemercaptoalkoxyalkyl-functional polysiloxanes having varied properties.The preferred synthesis, described below, requires only two steps andmay be carried out in a single reaction vessel, however it will berecognized that alterations in the synthesis procedure can be readilydevised which are within the scope of the present invention.

The preferred mercaptoalkoxyalkyl-functional polymers of the presentinvention are prepared from a dialkylhydrogen-chainstoppedpolydialkyl-alkylhydrogen siloxane copolymer. Such SiH-containingpolysiloxanes will ordinarily have the general formula: ##STR1## inwhich each R represents, independently, a monovalent alkyl radical offrom 1 to 8 carbon atoms, preferably methyl, and x+y is an integer ofabout 25 to 600 such that the polysiloxane has a viscosity of about 20to 5000 centipoise at 25° C., preferably 100 to 500 centipoise. Suchhydrogen-functional siloxane fluids are primarily linear and willtherefore have an R to Si ratio of approximately 2 to 1. These siloxanefluids will ordinarily have from about 0.5 to 50 percent by weighthydrogen-siloxy functionality and are made by processes well known inthe art. Minor amounts of mono- and tri-functional siloxane units, someof which may also contain hydrogen, may also occur in these fluids butwill not seriously detract from their usefulness.

The SiH-containing polysiloxanes are reacted with an ω-halo-alkene,preferably an ω-chloro-1-alkene such as allyl chloride, methallylchloride, 4-chloro-1butene, 10-chloro-1-decene, and other analogousunsaturated halogen-containing hydrocarbons. Mixtures of suchω-halo-alkenes will also be useful.

The alkene component and the hydrogen-functional polysiloxane componentare reacted in a precious metal-catalyzed addition cure reaction. Suchcatalysts are well known in the silicone art and will ordinarily be aplatinum metal complex effective to promote the addition of a .tbd.SiHmoiety to the double bond of an alkene. Examples of such hydrosilationcatalysts suitable for the purposes herein are described in U.S. Pat.No. 3,220,972 (Lamoreaux), U.S. Pat. No. 3,715,334 (Karstedt), U.S. Pat.No. 3,775,452 (Karstedt) and U.S. Pat. No. 3,814,730 (Karstedt), all ofwhich are incorporated herein by reference.

The product of the hydrosilation reaction described above is finallyreacted with a hydroxy-functional thiol monomer in the presence of anamine. Suitable thiols have the general formula HO--R³ --SH in which R³is divalent alkylene of from 2 to 20 carbon atoms or divalent alkylketone radicals, --(CH₂)_(n) C:O--, of from 2 to 20 carbon atoms.Preferred compounds are β-mercaptoethanol and γ-mercaptopropionic acid.For the purposes of the present invention, sufficient amounts of thehydroxy-functional thiol monomer should be used to provide amercaptoalkoxyalkyl-functional polysiloxane having from 0.5 to 50percent by weight mercaptoalkoxyalkyl siloxy functionality. Theabove-described synthesis may be illustrated as follows: ##STR2##

UV-curable mercaptoalkoxyalkyl-functional silicone compositions can bemade by combining the above-described mercaptoalkoxyalkyl-functionalpolysiloxanes with a catalytic amount of a photoinitiator. Any of thenumerous photocatalysts known to promote a curing, i.e., crosslinking,reaction between the mercaptofunctional group and the vinyl-functionalgroup of a vinyl-containing crosslinking compound, many of whichphotocatalysts are discussed in the previously cited U.S. Patents (whichare incorporated herein by reference), are suitable. Especiallycontemplated are the acetophenone-type photoinitiators such asdimethylhydroxyacetophenone, which is commercially obtainable under thetradename Darocure® 1173 (E.M. Chemicals, Inc.) A preferred feature ofthe present invention, however, stems from the discovery that certainperbenzoate esters having the general formula: ##STR3## where R⁴ is amonovalent alkyl or aryl group and Z is H, alkoxy, alkyl, halogen,nitro, amino, primary and secondary amino, amido, etc. The nature of theZ substituent will affect the stability of the peroxy bond, anelectron-poor substituent stabilizing the peroxy bond, and anelectron-rich substituent making the peroxy bond more reactive. Theseperbenzoate esters may be synthesized in known ways, such as by reactingbenzoyl halides with hydroperoxides. (See, e.g., the descriptions inBlomquist and Berstein, J. Amer. Chem. Soc., 73,5546 (1951)). Preferredperbenzoate esters include t-butylperbenzoate and its para-substitutedderivatives, t-butylper-p-nitrobenzoate, t-butylper-p-methoxybenzoate,t-butylper-p-methylbenzoate, and t-butylper-p-chlorobenzoate.T-butylperbenzoate is most preferred.

The amount of photoinitiator employed is not critical, so long as propercrosslinking is achieved. As with any catalyst, it is preferable to usethe smallest effective amount possible; however, for purposes ofillustration, catalyst levels of from about 1% to 5% by weight of thetotal composition have been found suitable.

In addition to the discovery that t-butylperbenzoate (and itsderivatives) is an excellent photocatalyst for radical addition ofmercaptofunctional siloxanes to vinyl-functional siloxanes, it has beendiscovered that the effectiveness of t-butylperbenzoate as aphotocatalyst is considerably enhanced when it is combined with certainphotosensitizers soluble in the silicone polymers of the the instantinvention. The use of these photosensitizers leads to advantages interms of release performance, ease of processing and lowering costs. Thephotosensitizers more than double the rate of cure in photoactivecompositions under inert conditions and, suprisingly, promotes good curewithout inerting, which allows important cost and processing advantages.

The photosensitizers are polyaromatic compounds having at least twobenzene rings which may be fused or bridged by organic radicals orhetero-radicals such as oxa, thio, etc. Preferred among thephotosensitizers tested were benzophenone and t-butylanthraquinone.Anthracene and thioxanthone were unsuccessful as photosensitizers due totheir limited solubility in silicone solutions. Other photosensitizercompounds related to those already mentioned will suggest themselves topersons skilled in the art and are meant to be included within the scopeof the present invention.

In the course of trials testing the effectiveness of the aforementionedperbenzoate esters and photosensitizers, previously unknownphotoreactive terpolymers were discovered which are capable of curing(on exposure to UV radiation) to abhesive compositions in the presenceof certain radical photosensitizers and without the use of perbenzoateesters. The terpolymers are mixed dimethylvinyl- andtrimethyl-chainstopped linear polydimethyl-methylvinyl-methylhydrogensiloxane terpolymer fluids and can be synthesized by acid equilibrationof methylhydrogen siloxane fluid,tetramethyltetravinylcyclotetrasiloxane (methylvinyltetramer) andoctamethylcyclotetrasiloxane (dimethyltetramer), as described morecompletely in the working examples (infra).

The UV-curable silicone compositions which may be prepared from thepreviously described ingredients can be applied to cellulosic and othersubstrates including paper, metal, foil, glass, polyethylene coatedkraft (PEK), supermetal, calendered kraft paper (SCK), polyethylenefilms, polypropylene films and polyester films. A photoinitiatedreaction will cure the silicone compositions to form an abhesive surfaceon the coated substrate. Inerting of the cure environment, such as withnitrogen, may be desirable where the presence of oxygen is found toinhibit the curing reaction

In order that persons skilled in the art may better understand thepractice of the present invention, the following examples are providedby way of illustration, and not by way of limitation. All measurementsare parts by weight.

EXAMPLES 1-10 Sample A

200 parts by weight of a 90 cps dimethylhydrogen-chainstopped linearpolydimethyl-methylhydrogen siloxane fluid having about 9.5% by weightSiH-containing siloxy units (about 0.31 moles SiH total) were combinedwith 200 parts by weight toluene and about 31.5 parts by weightmethallyl chloride (about 0.36 moles). A small amount of a platinumcatalyst was added and the reaction mixture refluxed at 110° C. for 14hours, at which time infrared examination detected no unreacted SiHfunctionality. Excess methallyl chloride was removed by distilling about6 parts by weight of solvent from the mixture at one atmospherepressure. 33 parts by weight γ-mercaptopropionic acid (0.31 moles) wereadded to the reaction vessel and a nitrogen atmosphere established priorto dropwise addition of 40 parts by weight triethylamine at atemperature of 35° C. A hazy percipitate (aminehydrochloride) formed asthe triethylamine was added. The solvent was stripped under a vacuum(about 5 mm pressure) at 158° C. for 30 minutes. Filtering the reactionproduct resulted in 173 parts by weight of a hazy fluid, 250 cpsviscosity.

Sample B

Another mercaptofunctional polysiloxane material was prepared in thesame manner as Sample A except that β-mercaptoethanol (0.31 moles) wassubstituted for γ-mercaptopropionic acid. 182 parts by weight of aslightly hazy 190 cps fluid product were obtained.

Sample C

250 parts by weight of a 50 cps SiH-containing fluid similar to thatused in Samples A & B containing about 6.0 percent by weightSiH-containing siloxy units (0.25 moles total SiH) were reacted withabout 27 parts by weight methallyl chloride by refluxing in 250 parts byweight toluene for 16 hours in the presence of a platinum catalyst.After removal of excess methallyl chloride, 500 parts by weight hexaneand 19.5 parts by weight (0.25 moles) β-mercaptoethanol were added,followed by dropwise addition of 20 parts by weight pyridine (0.26moles) under nitrogen at 60° C. The precipitate resulting from the amineaddition was filtered from the solution and the filtrate stripped ofsolvent and unreacted mercaptoethanol under a vacuum at 165° C. for 11/2hours. 226 parts by weight of a clear, pale yellow 130 cps product wereobtained.

Sample D

250 parts by weight of a 295 cps SiH-containing fluid having about 7.0percent by weight SiH-containing siloxy units (0.29 moles total SiH)were reacted with methallyl chloride, then β-mercaptoethanol andpyridene precisely as in the preparation of Sample C, above. Strippingthe reaction product at 165° C. for 4 hours yielded 237 parts by weightof a clear, pale yellow fluid product, 830 cps viscosity.

It was noted that the prolonged strip cycle at temperatures above about160° C. accomplished the removal of free (unreacted) mercaptoethanol(leaving an oder-free product) and the removal of residual pyridinehydrochlorate precipitate by vacuum sublimation (leaving a clear fluidproduct).

The mercaptoalkoxyalkyl-functional silicones produced in the abovefashion were used to produce ultraviolet radiation-curable releasecompositions, as set forth below:

    ______________________________________                                        Coating Composition 1:                                                                        10 parts Sample B                                                             1 part methylvinyltetramer                                                    (tetramethyl-tetravinyl                                                       cyclotetrasiloxane)                                                           0.5 part dimethylhydroxyaceto-                                                phenone (Darocure ® 1173).                                Coating Composition 2:                                                                        8 parts Sample B                                                              2 parts methylvinyltetramer                                                   0.5 part dimethylhydroxyaceto-                                                phenone                                                       Coating Composition 3:                                                                        8 parts Sample B                                                              2 parts vinyl terminated                                                      dimethylpolysiloxane                                                          fluid, 200 cps.                                                               0.5 part dimethylhydroxyaceto-                                                phenone.                                                      Coating Composition 4:                                                                        7 parts Sample D                                                              3 parts sym-tetramethyldivinyl-                                               disiloxane                                                                    0.4 part dimethylhydroxyaceto-                                                phenone.                                                      Coating Composition 5:                                                                        5 parts Sample D                                                              3 parts sym-tetramethyldivinyl-                                               disiloxane                                                                    2 parts vinyl terminal dimethyl-                                              methylvinyl siloxane fluid,                                                   200 cps                                                                       0.4 part dimethylhydroxyaceto-                                                phenone.                                                      ______________________________________                                    

Each coating composition was coated onto 40-lb. SCK paper as a thin filmwith a doctor blade, then exposed to ultraviolet radiation from twofocused mediumpressure mercury vapor lamps each operating at 300 wattsper square inch mounted in a PPG 1202 QC-AN processor. Exposure timesand curing atmosphere were varied to assess the cure performance of theexperimental compositions. The cure was qualitatively determined bynoting the presence or absence of smear, migration, or rub-off, with thefollowing results:

    ______________________________________                                        Coating       Exposure                                                        Com-   Cure   Time,                                                           position                                                                             ATM    Sec.     Qualitative Cure                                       ______________________________________                                        1      N.sub.2                                                                              1.5      No smear, no migration, no rub-off                     1      Air    1.5      Slight smear, slight migr., no rub-off                 1      N.sub.2                                                                              0.3      No migration, easy rub-off                             1      Air    0.3      Slight migration, easy rub-off                         1      N.sub.2                                                                              0.07     No migration, easy rub-off                             1      Air    0.07     Migrates, rubs off easily                              2      N.sub.2                                                                              0.3      No migration, rubs off easily                          2      Air    0.3      Slight migr., rubs off easily                          3      N.sub.2                                                                              0.3      Slight migr., rubs off easily                          3      Air    0.3      NOT CURED                                              4      N.sub.2                                                                              1.5      No smear, no migration, no rub-off                     4      Air    1.5      Slight smear, no migration, slight                                            rub-off                                                5      N.sub.2                                                                              1.5      No smear, no migration, no rub-off                     5      Air    1.5      Migrates, no rub off                                   5      N.sub.2                                                                              0.3      No migration, easy rub off                             ______________________________________                                    

These data indicate that a wide range of vinyl-functional siloxanes aresuitable crosslinkers in the UV cure of mercaptoalkoxyalkyl-functionalpolysiloxanes. Also, it is apparent that better results are obtainedunder a nitrogen atmosphere than in air, which is predictable given theradical-initiated nature of the curing reaction. Fully cured(migration-free) coatings are observed with exposure times as brief as1.5 seconds, although anchorage to the SCK paper (evidenced by rub-off)suffers at shorter exposure times.

It has been previously reported, in U.S. Pat. No. 4,139,385 (Crivello),incorporated herein by reference, that onium salt photocatalysts promotepolyolefin-polythiol cross-linking; and the following coatingcompositions were prepared to test the effectiveness of cationicphotocatalysts with the polymers of the present invention:

    ______________________________________                                        Coating Composition 6:                                                                      10 parts Sample B                                                             1 part methylvinyltetramer                                                    0.2 part (C.sub.12 H.sub.25 Ph).sub.2 ISbF.sub.6                              (bis(dodecylphenyl) iodonium                                                  hexafluoroantimonate photocatalyst).                            Coating Composition 7:                                                                      10 parts Sample C                                                             1 part methylvinyltetramer                                                    0.5 part dimethylhydroxyaceto-                                                phenone                                                                       0.2 part (C.sub.12 H.sub.25 Ph).sub.2 ISbF.sub.6.               Coating Composition 8:                                                                      7.5 parts Sample C                                                            0.5 part methylvinyltetramer                                                  2 parts 1,2-epoxydodecane*                                                    0.4 part dimethylhydroxyaceto-                                                phenone                                                                       0.2 part (C.sub.12 H.sub.25 Ph).sub.2 ISbF.sub.6.               ______________________________________                                         *w-epoxy monomer added as a cure enhancer, (described as to epoxysilicone     in copending U.S. Application Serial No. 375,676 filed May 6, 1982 and        incorporated herein by reference).                                       

The performance of these compositions was tested on supercalenderedkraft paper as in Examples 1-5 with the following results:

    ______________________________________                                        Com-          Exposure,                                                       position                                                                             Atm.   Sec.     Qualitative Cure                                       ______________________________________                                        6      Air    1.5      No smear, no migration, no rub-off                     6      N.sub.2                                                                              0.3      No smear, no migration, rubs-off                       7      Air    0.3      Slight smear, no migration, some                                              rub-off                                                7      N.sub.2                                                                              0.3      No smear, no migration, easy rub off                   8      Air    1.5      No smear, no migration, no rub-off                     8      N.sub.2                                                                              0.3      No smear, no migration, easy rub-off                   ______________________________________                                    

using the 'onium salt catalyst in this system allows those skilled inthe art to add an epoxy monomer reactive diluent (such as the linearepoxide present in Composition 8) to thevinylsiloxane-mercaptoalkoxyalkylsiloxane mixture in order to assistanchorage or modify the release characteristics of the cured coating.

The quantitative release performance was determined for two coatingcompositions by preparing coating baths as follows: (The Samplecompositions were dispersed in solvent in order to obtain even, lowsilicone depositions)

    ______________________________________                                        Bath 9:     20 parts Sample B                                                             2 parts methylvinyltetramer                                                   1 part dimethylhydroxyacetophenone                                            80 parts hexane.                                                  Bath 10:    20 parts Sample B                                                             2 parts methylvinyltetramer                                                   0.4 part (C.sub.12 H.sub.25 Ph).sub.2 ISbF.sub.6                              80 parts hexane.                                                  ______________________________________                                    

These baths were used to coat SCK paper using a #3 wire-wound rod, andthe coated papers were then exposed to UV radiation for 0.3 seconds asdescribed above. Laminates of the cured coatings were prepared byapplying a 10 mil coating of Monsanto Gelva® 263 agressive acrylicadhesive on top of the cured silicone coating and then pressing anuncoated sheet of SCK paper onto the adhesive. 2"×9" strips of thelaminates were cut, and the silicone/SCK lamina separated from theadhesive/SCK lamina at a 180° angle at 400 ft./minute in a Scott tester.The release, in grams force needed to separate the two lamina, wasrecorded and the following results obtained:

    ______________________________________                                                         Qualitative                                                  Bath   Atm.      Cure         Release grams                                   ______________________________________                                         9     Air       No migr., rubs-off                                                                         150-190                                          9     N.sub.2   No migr., rubs-off                                                                         350-450                                         10     Air       (poor cure)  (not recorded)                                  10     N.sub.2   No migr., rubs-off                                                                         150-210                                         ______________________________________                                    

Although the cured silicone release coatings did not appear to be wellanchored, as evidenced by rub-off, there was no observation of thesilicone layer being pulled away preferentially from the SCK substrate.The results also show a high release (as opposed to "premium" release,usually <100 grams), especially when cured in an inert atmosphere,suggesting that the radicals formed in the silicone coating interactwith the acrylic monomers present in the adhesive to raise the observedrelease

EXAMPLES 11 & 11A

The efficacy of t-butylperbenzoate as a photocatalyst for thiol/vinyladdition was demonstrated in the following comparative trial:

    ______________________________________                                        Coating Composition 11:                                                                       9.0 parts Sample D (described                                                 previously)                                                                   1.0 parts methylvinyltetramer                                                 0.5 part t-butylperbenzoate.                                  Coating Composition 11A:                                                                      9.0 parts Sample D                                                            1.0 part methylvinyltetramer                                                  0.5 parts dimethylhydroxyaceto-                                               phenone (Darocure ® 1173).                                ______________________________________                                    

These coating compositions were hand-coated on 40-lb. supercalenderedkraft paper using a doctor blade, exposed to ultraviolet light asdescribed above, and finally qualitatively assessed for cure as arelease surface, as summarized below:

    ______________________________________                                                     UV                                                               Ex-          Exposure,                                                        ample Atm.   sec.      Qualitative Cure                                       ______________________________________                                        11 .sup.                                                                            AIR    0.6       Slight smear, no migration, slight                                            rub off                                                11 .sup.                                                                            N.sub.2                                                                              0.3       No smear, no migration, no rub-off                     11 .sup.                                                                            N.sub.2                                                                              0.15      No smear, no migration, slight                                                rub-off                                                11A   AIR    1.5       Smears, no migration, easily                                                  rubbed-off                                             11A   N.sub.2                                                                              1.5       Slight smear, no migration, slight                                            rub-off                                                11A   N.sub.2                                                                              0.3       Smears, no migration, easily rubbed-                                          off                                                    ______________________________________                                    

From these data, it is apparent that t-butylperbenzoate is superior tothe commercially available Darocure® 1173 photoinitiator in this curablemercapto-vinyl silicone system.

EXAMPLES 12-14 & 14A

Further investigation into the photocatalytic qualities of perbenzoateesters was conducted by preparing the following coating compositions:

    ______________________________________                                        Coating Composition 12:                                                                       10 parts vinyl terminal dimethyl                                              fluid*                                                                        0.6 part trimethyl terminal                                                   methylhydrogen fluid**                                                        0.5 part t-butylperbenzoate.                                  Coating Composition 13:                                                                       10 parts vinyl terminal dimethyl                                              fluid                                                                         1.0 part trimethyl terminal                                                   methylhydrogen fluid                                                          0.5 part t-butylperbenzoate                                                   0.5 part dimethylhydroxyaceto-                                                phenone.                                                      Coating Composition 14:                                                                       10 parts vinyl-functional                                                     terpolymer.sup.3                                                              0.5 part t-butylperbenzoate.                                  Coating Composition 14A:                                                                      10 parts vinyl-functional                                                     terpolymer                                                                    0.5 part 2,5-bis(t-butylperoxy)                                               hexane (control)                                              ______________________________________                                         *Dimethylvinyl terminal dimethyl polysiloxane, 200 cps viscosity fluid.       **Trimethyl terminal methylhydrogen polysiloxane, about 30 cps viscosity      fluid. .sup.3 Dimethylvinyl terminal dimethylmethylvinyl-methylhydrogen       polysiloxane fluid, ratio of dimethyl:methylvinyl:methylhydrogen units        equals, approximately, 88:5:6, 8 0 cps viscosity fluid.                  

Because t-butylperbenzoate may be used to promote thermally-activatedcrosslinking reactions, the control composition, 14A, was included inthe trial. 2,5-bis(t-butylperoxy) hexane has similar thermal activity tot-butylperbenzoate, so that if the heat generated by the UV curing lampswas responsible for curing the coating compositions (instead of theultraviolet radiation), then the curing characteristics of compositions14 and 14A would be similar.

The cure performance in terms of exposure time required to achieve atack-free release surface was tested by the following method:

Each mixture was applied as a thin film to 40-lb. supercalendered kraft(SCK) paper by hand with a doctor blade, then exposed to two Hanoviamedium pressure mercury vapor ultraviolet lamps, each operating at 300watts per square inch focused power, mounted in a PPG QC 1202 ANprocessor. The following results were observed:

    ______________________________________                                                                  Tack-Free Exposure                                  Coating Composition                                                                         Atmosphere  time, seconds                                       ______________________________________                                        12            INERT       0.9                                                 12            AIR         No Cure (5.0 sec.)                                  13            INERT       1.5                                                 13            AIR         No Cure (5.0 sec.)                                  14            INERT       0.3                                                 14            AIR         No Cure (5.0 sec.)                                   .sup. 14A    INERT       No Cure (5.0 sec.)                                   .sup. 14A    AIR         --                                                  ______________________________________                                    

Failure of the control composition to cure even in an inert cureenvironment indicates that t-butylperbenzoate is a photocatalyst for awide range of vinyl-hydrogen crosslinking reactions. The cured coatingcompositions also exhibited typical release surface characteristics,with no migration to Scotch® #610 adhesive tape but with a slighttendency to smear. Poor cure in the presence of oxygen indicates theradical nature of the crosslinking reaction. It is believed that theperoxy group bonded directly to the phenyl ketone chromophore throughthe carbonyl carbon atom is necessary for photoactivity.

EXAMPLES 15-25

Derivatives of t-butylperbenzoate were synthesized to evaluate theirperformance as photocatalysts.

P-nitro Perbenzoate Derivative

33.4 parts by weight of 70% aqueous t-butylhydroperoxide were placed ina reaction vessel and cooled in an ice bath. 65 parts by weight of a 30%aqueous solution of KOH and 46.4 parts by weight ofp-nitrobenzoylchloride dissolved in 90 parts by weight CH₂ Cl₂ weresimultaneously added to the vessel slowly over a 30-minute period. Thisreaction mixture was stirred for 2 hours at 0° C., and then for 3 hoursat 25° C. The organic layer was separated from the aqueous layer,transferred to a separatory funnel and twice washed with a 5% Na₂ CO₃solution, thrice washed with water and finally dried over anhydrousMgSO₄. Removal of methylene chloride under a vacuum furnished 30.9 partsby weight of a crude yellow product, which was recrystallized fromacetone/hexane to furnish yellow crystalline t-butylper-p-nitrobenzoate.This was designated D_(n), or p-nitro derivative.

P-methoxy Perbenzoate Derivative

Synthesis of t-butylper-p-methoxybenzoate was carried out in the samemanner as the p-nitro derivative. Since the product is a liquid at roomtemperature, purification was achieved by dissolving the crude productin CH₃ CN, then extracting twice with hexane. The product was obtainedfrom the acetonitrile layer by removing CH₃ CN under a vacuum, to give a64% yield of a clear, pale yellow, viscous fluid. This product wasdesignated D_(m), or p-methoxy derivative.

P-tolyl Perbenzoate Derivative

T-butylper-p-methylbenzoate was synthesized and purified precisely asthe p-methoxy derivative. Purification gave an 85% yield of a clear,colorless fluid product which was designated D_(t), or p-tolylderivative.

P-chloro Perbenzoate Derivative

T-butylper-p-chlorobenzoate was synthesized according to the procedureused for the p-nitro derivative. The tendency of the compound tosupercool resulted in the recovery of a product with a melting point ofapproximately 10°-30° C., compared to 49° C. for this compound knownfrom the literature. The solid product slowly melted at room temperatureto a clear, pale yellow viscous fluid. No further effort was made topurify this sample, which was designated D_(c), or p-chloro derivative.

In addition to the foregoing syntheses, a number of reactive siliconeterpolymers were prepared by acid-catalyzed (Filtrol® 20, acid treatedplay) equilibration of trimethyl-chainstopped methylhydrogenpolysiloxane, methylvinyltetramer, and dimethyltetramer.Photosensitizers were added to the terpolymers to enhance cure,according to the discovery discussed previously and demonstrated inExamples 25-56, supra. These terpolymer compositions are describedbelow:

    ______________________________________                                        Silicone Terpolymer Compositions                                                       Wt. %   Wt. %    Wt. %   Photosensi-                                 Terpolymer                                                                             D units D.sup.H units                                                                          D.sup.vi units                                                                        tizer Wt. %                                 ______________________________________                                        A        75.0    20.0     5.0     AQ, 0.2                                     B        77.0    11.3     11.7    BP, 2; AQ, 0.2                              C        88.0    10.0     2.0     BP, 2; AQ, 0.25                             D        48.0    50.0     2.0     BP, 2; AQ, 0.25                             E        45.0    50.0     5.0     BP, 2; AQ, 0.25                             F        45.0    50.0     5.0     BP, 3; AQ, 0.3                              ______________________________________                                         D = dimethyl siloxy units                                                     D.sup.H = methylhydrogen siloxy units                                         D.sup.vi = methylvinyl siloxy units                                           AQ = 2t-butylanthraquinone                                                    BP = benzophenone                                                        

The relative UV-curing performance of the various para-substitutedperbenzoate derivatives was determined by assessing the the speed andquality of cure of a photo-sentitized silicone terpolymer combined withvarious perbenzoate ester photocatalysts. Low solubility of some of theperbenzoate derivatives necessitated coating and curing the compositionsout of solvent.

Two coating baths were prepared, as follows:

    ______________________________________                                        Bath 15:        20 pbw terpolymer C                                                           80 pbw hexane                                                                 1 pbw t-butylperbenzoate                                      Bath 16:        20 pbw terpolymer C                                                           80 pbw hexane                                                                 1 pbw D.sub.n.                                                ______________________________________                                    

The coating mixtures were applied to SCK substrates with a #3 wire-woundrod. Coated samples were exposed to ultraviolet lamps as 600 watts persquare inch under inert conditions in a PPG QC 1202 AN processor aspreviously described until smear- and migration-free abhesive surfaceswere obtained. The unsubstituted perbenzoate-catalyzed composition (No.15) cured after 0.6 seconds UV exposure; the D_(n) -containingcomposition required 5.0 seconds UV exposure for the same degree ofcure.

Another set of coating baths were prepared as follows:

    ______________________________________                                                                 Perbenzoate                                          Baths    Terpolymer      Derivative                                           ______________________________________                                        17       D               D.sub.t, 5 Wt. %                                     18       D               D.sub.m, 5 Wt. %                                     19       D               D.sub.c, 5 Wt. %                                     20       D               Control, 5 Wt. %                                     ______________________________________                                         Control = tbutylperbenzoate (unsubstituted)                              

Of these solventless compositions, only the p-methoxy derivative, D_(m),formed an opaque mixture in terpolymer D, the other mixtures remainedclear. The compositions were hand-coated on SCK substrates with a doctorblade, then cured under inert and ambient (air) environments as inExamples 15 & 16, to give the following results:

    ______________________________________                                        Minimum UV Exposure Required for Cure                                          ##STR4##                                                                     in Terpolymer D                                                               Derivative  R        Atm.     Cure time, sec.                                 ______________________________________                                        Control     H        AIR      1.5                                             Control     H        INERT    0.3                                             D.sub.c     Cl       AIR      1.5                                             D.sub.c     Cl       INERT    0.3                                             D.sub.t     Me       AIR      0.6                                             D.sub.t     Me       INERT    0.15                                            D.sub.m     OMe      AIR      0.6                                             D.sub.m     OMe      INERT    0.15                                            ______________________________________                                    

From these data is appears that the perbenzoate ester catalysts may bequalitatively ranked (in ascending order of activity): D_(n) (very lowactivity), D_(c) roughly equal to unsubstituted t-butylperbenzoate, andD_(t) roughly equal to D_(m). This rank roughly corresponds tothermally-induced peroxy bond dissociation energies observed in theliterature. It is believed that higher alkoxy derivatives, e.g.,P-butoxy, p-ethoxy, or p-dodecyloxy, etc. might overcome the solubilityproblems of the p-methoxy derivative, D_(m).

It was a further discovery during the course of these trials that thephotosensitized silicone terpolymers described above were capable ofphotocuring to abhesive coatings without the assistance of a perbenzoateester photoinitiator.

Solvent-free coating baths were prepared as follows:

    ______________________________________                                        Bath         Composition                                                      ______________________________________                                        21           Terpolymer F alone (contains                                                  photosensitizers: 3 Wt. % BP                                                  and 0.3 Wt. % AQ)                                                22           10 pbw terpolymer F + 0.5 pbw                                                 diethoxyacetophenone (DEAP)                                      23           10 pbw terpolymer F + 0.5 pbw                                                 Trigonal ® 14 (mixture of                                                 isobutyl benzoin esters;                                                      Noury Chemical Co.)                                              24           10 pbw terpolymer F + 0.5                                                     pbw t-butylperbenzoate.                                          25           10 pbw terpolymer F + 0.5                                                     pbw DEAP + 0.5 t-butylper-                                                    benzoate.                                                        ______________________________________                                    

Baths 21-24 were clear fluids, while bath 25 was hazy and partiallyopaque due to the limited solubility of DEAP and t-butylperbenzoatemixtures in non-polar silicone fluids.

The baths were coated on SCK substrates and cured as in Examples 17-20,and the minimum UV exposure time until smear- and migration-freecoatings were achieved was recorded

    ______________________________________                                                              Minimum UV                                              Bath    Cure Atmosphere                                                                             Exposure for Cure, sec.                                 ______________________________________                                        21      AIR           1.5                                                     21      INERT         0.6                                                     22      AIR           0.3                                                     22      INERT         0.3                                                     23      AIR           1.5                                                     24      AIR           1.3                                                     24      INERT         0.15                                                    25      AIR           0.3                                                     25      INERT         0.3                                                     ______________________________________                                    

These date suggest that H-abstraction from the Si-H bond by thephotosensitizers is involved in the curing mechanism. Benzophenone andt-butylanthraquinone are known to produce free radicals followingexcitation by ultraviolet radiation in the presence of a suitable protondonor.

Combinations of photosensitizers with photoinitiators capable ofgenerating radical pairs via unimolecular homolysis afterphotoexcitation are reported to be a means of minimizing oxygenquenching of radical-induced crosslinking by Gruber, U.S. Pat. No.4,071,652 (incorporated herein by reference), and this effect is evidentin comparing the performance of Baths 21 and 22, where DEAP is thephotoinitiator generating radical pairs. Also noted is a synergisticeffect in combining photosensitizers with perbenzoate photoinitiators,when comparing the performance of Baths 21, 24 and 25. Althoughsolubility problems evidently interfered with the performance of Bath E,it is believed that enhanced cure will result from combinationsincluding benzophenone, t-butylanthraquinone, DEAP andt-butylperbenzoate in a mutually compatable medium.

EXAMPLES 26-56

The following compositions were prepared to show the effect of certainpolyaromatic photosensitizers:

880 parts by weight dimethyltetramer, 50 parts by weightmethylvinyltetramer, 20 parts by weight ofsym-tetramethyldivinyldisiloxane, and 63 parts by weight oftrimethyl-chainstopped polymethylhydrogensiloxane fluid (30 cps) wereblended together with 5 parts by weight of an acid clay catalyst(Filtrol® 20), then agitated under a nitrogen atmosphere at 60° C. for15 hours. Removing the catalyst by filtration afforded a mixeddimethylvinyland trimethyl-chainstopped linearpolydimethyl-methylvinylmethylhydrogensiloxane terpolymer fluid (70cps), which was designated Sample G.

300 parts by weight of Sample G were stripped of light ends under avacuum at 165° C. for 2 hours. 241 parts by weight of the product weretreated with 1.2 parts by weight benzophenone and the mixture stirred at100° C. for 30 minutes until a clear solution was obtained. Thebenzophenone remained in solution when the polymer was cooled to roomtemperature, to give a 340 cps fluid product designated Sample H.

Another terpolymer was prepared exactly as Sample G from the followingmaterials: 1260 parts by weight dimethyltetramer, 92 parts by weightmethylvinyltetramer, 15 parts by weightsym-tetramethyldivinyldisiloxane, and 150 parts by weight of themethylhydrogen fluid. Filtering and stripping the equilibrate afforded1240 parts by weight of a 1225 cps fluid, designated Sample J.

A composition designated Sample K was prepared consisting of 0.5 weightpercent solution of benzophenone in the Sample J terpolymer.

A composition designated Sample L was prepared consisting of a 1.0weight percent solution of benzophenone in the Sample J terpolymer.

A composition designated Sample M was prepared consisting of a 2.0weight percent solution of benzophenone in the Sample J terpolymer.

A composition designated Sample N was prepared consisting of a 4.0weight percent solution of benzophenone in the Sample J terpolymer.

A composition designated Sample P was prepared consisting of a 0.2weight percent solution of 2-t-butylanthraquinone in the Sample Jterpolymer.

It was noted that the benzophenone was quite soluble in silicone fluidsto at least 4 weight percent. The limits of solubility of the chemicallysimilar t-butylanthraquinone have not been established, however, simpleexperimentation will reveal to the persons skilled in this art theuseful range of solubility for this and other photosensitizers suitablefor the purposes disclosed herein. Attempts to prepare 0.5 weightpercent solutions of anthracene and thioxanthone in the Sample Jterpolymer were unsuccessful due to their limited solubility insilicones.

Ultraviolet cure analyses were performed using Samples G-P in thefollowing manner. Each sample was mixed with 5 weight percentp-butylperbenzoate, manually coated as thin films on 40-lb.supercalendered kraft (SCK) paper with a doctor blade, then passedthrough a PPG QC 1202 An ultraviolet processor (2 Hanoviamedium-pressure mercury vapor UV lamps each capable of operating at 100,200 or 300 watts per square inch focused power). The degree of cure wasqualitatively assessed by noting the presence or absence of smear,migration, and rub-off in irradiated coatings. After curing the testcoatings prepared from Samples G-P under various cure conditions, thefollowing results were obtained:

    ______________________________________                                        Ex-         Total lamp                                                                              Exposure                                                am-  Sam-   power,    time,  Cure                                             ple  ple    watts     seconds                                                                              Atm. Qualitative Cure                            ______________________________________                                        26   G      600       0.3    N.sub.2                                                                            No smear, no migra-                                                           tion, slight rub-off                        27   G      600       1.5    AIR  NOT CURED                                   28   H      400       0.3    N.sub.2                                                                            No smear, no migra-                                                           tion, slight rub-off                        29   H      600       0.15   N.sub.2                                                                            No smear, no migra-                                                           tion, slight rub-off                        30   H      600       1.5    AIR  No smear, no migra-                                                           tion, no rub-off                            31   J      400       0.3    N.sub.2                                                                            No smear, no migra-                                                           tion, slight rub-off                        32   J      600       0.15   N.sub.2                                                                            No smear, no migra-                                                           tion, moderate                                                                rub-off                                     33   J      600       0.6    AIR  NOT CURED                                   34   K      200       0.6    N.sub.2                                                                            No smear, no migra-                                                           tion, slight rub-off                        35   K      400       0.1    N.sub.2                                                                            No smear, no migra-                                                           tion, moderate                                                                rub-off                                     36   K      600       0.08   N.sub.2                                                                            No smear, no migra-                                                           tion, easy rub-off                          37   L      200       0.3    N.sub.2                                                                            No smear, no migra-                                                           tion, moderate rub-off                      38   L      400       0.1    N.sub.2                                                                            No smear, no migra-                                                           tion, moderate                                                                rub-off                                     39   L      600       0.08   N.sub.2                                                                            No smear, no migra-                                                           tion, easy rub-off                          40   M      100       0.3    N.sub.2                                                                            No smear, no migra-                                                           tion, moderate                                                                rub-off                                     41   M      400       0.1    N.sub.2                                                                            No smear, no migra-                                                           tion, moderate                                                                rub-off                                     42   M      600       0.6    AIR  No smear, no migra-                                                           tion, no rub-off                            43   N      200       0.3    N.sub.2                                                                            No smear, no migra-                                                           tion, moderate                                                                rub-off                                     44   N      600       0.08   N.sub.2                                                                            No smear, no migra-                                                           tion, easy rub-off                          45   N      600       0.3    N.sub.2                                                                            No smear, no migra-                                                           tion, no rub-off                            46   N      600       0.3    AIR  No smear, no migra-                                                           tion, no rub-off                            47   P      200       0.3    N.sub.2                                                                            No smear, no migra-                                                           tion, moderate                                                                rub-off                                     48   P      600       0.3    N.sub.2                                                                            No smear, no migra-                                                           tion, no rub-off                            49   P      600       0.3    AIR  No smear, no migra-                                                           tion, no rub-off                            50   P      600       0.8    N.sub.2                                                                            No smear, no migra-                                                           tion, easy rub-off                          ______________________________________                                    

It is apparent from these results that the presence of benzophenone ort-butylanthraquinone more than doubles the rate of cure for thephotoactive compositions under inert conditions, and, surprisingly,permits good cure (at fairly fast rates) even without inerting the cureenvironment. The latter observation can be particularly important wherethe expense of providing an inert atmosphere for radiation curing is acritical concern of the particular user. The compositions anchor well toSCK substrates when UV lamps are operated at high intensity for at least0.3 seconds exposure. Shorter exposure provides migration-free adhesivecoatings which are easily rubbed off, although they are otherwise fullycured.

Several samples were next assessed for release performance againstcommon adhesives. The following coating baths were prepared(measurements are in parts by weight):

    ______________________________________                                        Bath Q:         20 pbw Sample J                                                               1 pbw t-butylperbenzoate                                                      79 pbw hexane.                                                Bath R:         20 pbw Sample L                                                               1 pbw t-butylperbenzoate                                                      79 pbw hexane                                                 Bath S:         20 pbw Sample M                                                               1 pbw t-butylperbenzoate                                                      79 pbw hexane                                                 Bath T:         20 pbw Sample N                                                               1 pbw t-butylperbenzoate                                                      79 pbw hexane                                                 Bath U:         20 pbw Sample P                                                               1 pbw t-butylperbenzoate                                                      79 pbw hexane                                                 ______________________________________                                    

Each of the baths Q-U were coated onto 9"×12" sheets of SCK paper with a#3 wire-wound rod, then exposed to ultraviolet light in the PPGultraviolet processor as previously described. The cured compositionswere then coated with a 10 mil layer of Gelva® 263 acrylic adhesive(Monsanto) and cured for 15 minutes at 65° C. An uncoated SCK sheet wasthen affixed to the adhesive layer. These laminates were cut intostrips, and the release in grams recorded as in previous examples, withthe following results:

    ______________________________________                                                    Total                                                             Ex-         lamp     Expo-                                                    am-         power,   sure, Cure Qualitative                                                                             Release,                            ple  Bath   watts    sec.  Atm. Cure      grams                               ______________________________________                                        51   Q      100      0.3   N.sub.2                                                                            No migration,                                                                           55-80                                                               poor anchorage                                52   R      100      0.3   N.sub.2                                                                            No migration,                                                                           50-85                                                               poor anchorage                                53   S      600      0.3   AIR  No migration,                                                                           210-250                                                             good anchorage                                54   S      100      0.3   N.sub.2                                                                            No migration,                                                                           60-90                                                               good anchorage                                55   U      600      0.3   AIR  No migration,                                                                           110-130                                                             good anchorage                                56   U      600      0.3   N.sub.2                                                                            No migration,                                                                           25-40                                                               good anchorage                                ______________________________________                                    

Examples 51, 52 and 54 indicate that benzophenone concentration haslittle effect on release. Although some of the cured compositions didnot appear to anchor to the substrate particularly well, the separatedlamina showed no evidence that the silicone layer adhered preferentiallyto the aggressive Gelva® adhesive

Examples 53, 55 and 56 indicate that the nature of the curingenvironment does have an effect that is not evident from the previousExamples 26-50: Air cure apparently leads to a higher release than cureof the same composition in an inert environment. This difference inrelease suggests that varying the amount of nitrogen in the curingchamber may provide a means of controlling the release of a siliconecomposition. It is also apparent that a low level on anthraquinone(0.2%) is more effective than benzophenone for promotingt-butylperbenzoatecatalyzed UV cure of the vinyl-functional terpolymersof the present invention.

Because t-butylperbenzoate is thermally very stable and can be storedindefinitely at room temperature, the compositions of the presentinvention can be packaged and sold as 1-part systems, eliminatingpreparatory steps and making the compositions easier to use.

EXAMPLES 57 & 58

Higher alkoxy-substituted perbenzoate photo-catalysts were synthesizedto try to overcome the solubility problems encountered with thep-methoxy and p-tolyl derivatives of t-butylperbenzoate:

T-butylper-p-butoxybenzoate was synthesized by first preparingp-butoxybenzoylchloride according to the procedure described by Rohrmannand Wischniewski, Arch. Pharm., 292, 787 (1959). 29.1 parts by weight of4-n-butoxybenzoate 4-n-butoxybenzoic acid and 71.4 parts by weightthionylchloride were mixed and agitated at 86° C. for 1 hour. Theresulting clear solution was stripped of excess thionylchloride under avacuum at 60° C., and the product isolated by vacuum distillation. 30.6parts by weight of clear viscous fluid (b.p. 143°-147° C./7mm) wereobtained (95° yield). This product was added slowly to 18 parts byweight of 70% aqueous t-butylhydroperoxide simultaneously with 35 partsby weight of 30% aqueous potassium hydroxide (also slowly added).Reaction temperature was maintained at 0°-5° C. during this addition.The p-butoxy derivative was isolated by dissolution in acetonitrile,extracting twice with hexane, and removal of the acetonitrile under avacuum, resulting in 26.2 parts by weight of a clear, pale yellow fluidproduct. The infrared spectrum of the product was consistent with thestructure of the p-butoxy derivative of t-butylperbenzoate.

A p-dodecyloxy derivative was prepared in the same manner as thep-butoxy derivative, using p-dodecyloxybenzoyl chloride. The finalproduct was a viscous pale yellow liquid at room temperature, and theinfrared spectrum was consistent with the structure of the p-dodecyloxyderivative.

Two silicone release coating compositions were prepared using the abovederivatives by mixing 5 parts by weight of each derivative,respectively, with 100 parts by weight of a 670 cps linearpolydimethyl-methylvinyl-methylhydrogen siloxane fluid with 7 weightpercent methylvinylsiloxy units, 5.2 weight percent methylhydrogensiloxy units and the remainder consisting of dimethylsiloxy units. Ineach case, an opaque white mixture resulted. Although both compositionscould be cured to smear- and migration-free release coatings on SCKsubstrates in 0.3 seconds UV exposure (at 600 watts/square inch), theapparent poor compatibility of these two derivatives with pure siliconeterpolymer solutions suggests that they are not the catalysts of choicefor solvent-free release applications.

EXAMPLES 59-63

Further trials were conducted with terpolymers prepared according to thepresent invention in order to assess cure characteristics and storagestability. Three new terpolymers having different Si-H/Si-vinyl ratioswere prepared as in previous examples and compared with terpolymer F(see Examples 15-25):

    ______________________________________                                        Ex-                                                                           am-  Ter-                       Mole Ratio                                    ple  polymer  Photosensitizer   Si--H/Si--vinyl                               ______________________________________                                        59   F        3 Wt. % BP, 0.3 Wt. % AQ                                                                        14.0                                          60   X        2 Wt. % BP        2.80                                          61   Y        2 Wt. % BP        1.50                                          62   Z        3 Wt. % BP        1.00                                          ______________________________________                                         BP = benzophenone                                                             AQ = tbutylanthraquinone                                                 

The release performance of the above terpolymers was assessed as inprevious examples, by preparing coating baths consisting of 20 parts byweight of a terpolymer, 0.6 pbw t-butylperbenzoate and 80 pbw hexane,applying each coating composition to SCK paper, curing under UV light at600 watts/square inch for 0.3 seconds (inert atmosphere), and finallypreparing laminates using 5-mil coatings of aggressive SBR adhesive(Coated Products, Inc. #4950) and recording the force required toseparate the lamina (400 ft./minute; 180° angle). Release was testedinitially and periodically after aging at 150° F. The following dataresulted:

    ______________________________________                                                         Release  Release                                                                              Release                                                                              Release                               Ex-              (grams)  (grams)                                                                              (grams)                                                                              (grams)                               ample Terpolymer Initial  1 day  1 week 2 weeks                               ______________________________________                                        59    F          30-50    >500   >500   >500                                  60    X          35-45    45-65  140-160                                                                              160-180                               61    Y          35-50    40-60   85-105                                                                               95-120                               62    Z          45-65    50-65  65-90  70-90                                 ______________________________________                                    

It is apparent that a large excess of unreacted Si-H functionalityrelative to Si-vinyl-functionality will affect release, probably due tointeraction of the Si-H function with the adhesives. The data suggestthat where stable release is desired, the Si-H/Si-vinyl ratio should beheld at lower levels, preferably below about 2/1 for the terpolymersdescribed herein.

A separate trial to assess storage stability of a 1-part 1UV-curablecomposition was run by preparing a coating composition using 100 pbw ofTerpolymer X (containing benzophenone, 2%) blended witht-butylperbenzoate (5 pbw). The composition was split into two samples,one sample being maintained at room temperature in the dark and theother at 66° C. also in the dark. The viscosity of these blends wasmonitored over time to obtain an indication of storage stability. Thefollowing results were observed:

    ______________________________________                                        Storage                                                                       Temp.   Initial 1 Week   2 Weeks                                                                              4 Weeks                                                                              8 Weeks                                ______________________________________                                        25° C.                                                                         560 cps  550 cps 540 cps                                                                              550 cps                                                                              550 cps                                66° C.                                                                         550 cps 4370 cps (gelled)                                                                             --     --                                     ______________________________________                                    

These data suggest that a storage temperature below about 30° C. isrecommendable to prevent cure during storage.

EXAMPLES 64 & 64A

A trial comparing the cure performance of reactive terpolymers preparedaccording to the present invention to conventional UV-curable,vinyl-functional systems was run as follows:

A linear polydimethyl-methylvinyl-methylhydrogen siloxane terpolymer wasprepared having a molar Si-H/Si-vinyl ratio of 2.8 and including 83weight percent dimethylsiloxy units was prepared in which 2.5 weightpercent benzophenone was dissolved. A conventional UV-curable,vinyl-functional silicone blend was prepared consisting of 100 parts byweight of a 200 cps dimethylvinyl-chainstopped linearpolydimethyl-methylvinyl siloxane fluid having 5.2 weight percentmethylvinyl siloxy units, combined with 3 parts by weight of a 30 cpstrimethyl-chainstopped linear polymethylhydrogen siloxane fluid, alsocontaining 2.5 parts by weight benzophenone.

The cure performance of the two coating blends was qualitatively testedas described in previous examples, with the PPG ultraviolet processoroperating with 400 watts/square inch total focused power. The minimum UVexposure time (seconds) to obtain a smear-and-migration-free releasecoating on SCK substrates was recorded:

    ______________________________________                                                                 Exposure Time                                        Composition   Curing Atm.                                                                              for Cure, sec.                                       ______________________________________                                        64 .sup.      N.sub.2    0.3                                                  64 .sup.      AIR        1.5                                                  64A           N.sub.2    3.0                                                  (control)                                                                     64A           AIR        No cure after                                        (control)                15 sec.                                              64 + 3% t-butyl-                                                                            N.sub.2    0.15                                                 perbenzoate                                                                   ______________________________________                                    

These data indicate that the instant terpolymer compositions aresuperior in cure efficiency to known vinyl-functional systems;additionally, there appears to be a synergistic effect from thecombination of benzophenone and perbenzoate catalysts, allowing linespeeds 20 times faster than those required to fully cure conventionalcoatings.

Modifications and variations in the present invention are obviouslypossible in light of the foregoing disclosure. For example, manypolyaromatic compounds soluble in a given silicone composition andeffective as photosensitizers for the purposes described herein willsuggest themselves to persons of ordinary skill in this art and may beutilized advantageously with coating compositions prepared in accordancewith the present disclosure. It is understood, however, that any suchincidental change made in the particular embodiments of the inventionare within the full intended scope of the appended claims.

What is claimed is:
 1. An ultraviolet radiation-curable silicone release coating composition comprising:(A) a photoreactive mixed dimethylvinyl- and trimethyl-chainstopped linear polydimethyl-methylvinylmethylhydrogensiloxane terpolymer, and (b) a small amount effective to promote photopolymerization of (i) a perbenzoate ester, (ii) a polyaromatic photosensitizer or combination of such photosensitizers, or a combination of (i) and (ii).
 2. A silicone release coating composition as defined in claim 1, wherein said terpolymer has a ratio of dimethyl: methylvinyl: methylhydrogen units equal to, approximately, 40-90: 1-20: 5-50.
 3. A photoreactive terpolymer capable of curing on brief exposure to ultraviolet radiation in the presence of a photoinitiator prepared by acid equilibration of trimethyl-chainstopped polymethylhydrogensiloxane, tetramethyl-tetravinylcyclotetrasiloxane, and octamethylcyclotetrasiloxane. 